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Old 24-08-2016, 13:02   #31
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Re: AIS Antenna

trantor, Paul, Weyalan, Danny, Ed, Jim, J Clark, et al...

Oh boy, there is a LOT of info to get thru and clarify!!!
And, if I did my "usual", this would take a while...
But, I'll try to be brief!!


1) First off, some are answering questions posted 4 years ago....let's assume those folks long ago got things worked out okay!
So, I'm only going to address the recent queries / comments...


2) Most importantly, when dealing with AIS and VHF radio range, you should ALL read this thread here, where you will find all the facts (and probably more than you'd ever care to know!), about the "how far" and "why's 'n wherefor's"....
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

PLEASE read that thread! (so, I don't have to re-type it all here!
You'll see that comms DO take place beyond "line-of-sight"...

But, FYI....if you wish to see what "line-of-sight" is....
Quote:
For nautical miles......
VHF Radio line-of-sight distance in nautical miles = 1.23 x √ Af (where Af is the height, in feet, of Antenna)

Line-of-sight communications range between two VHF radio stations (in nautical miles) = 1.23 x √ Af1 + 1.23 x √ Af2





3) The maximum radiation to/from an antenna takes place at the high-current point....and with 1/2-wave antennas, this is in the middle of the antenna...so, "theoretically" the line-of-sight distance can be calculated from using the height of the center/middle of the antenna...

Please understand that the slight variance between radio line-of-sight of 9' vs. 10', or 63' vs. 64', is SO small that it is almost irrelevant!!
But, since the question was asked, if you wish to be precise, we would use the height measured at the center of your VHF whip antenna...



4) Class A AIS transmits at 12.5 watts, not 25 watts...
Class B AIS transmits at 2 watts, not 3 watts....

The digital modulation/demodulation of AIS signals and their coding aren't quite as good as the human ear / human brain combo, when listening to very weak signals, so AIS signals theoretically don't have quite as much range as the FM Voice....BUT...
But, in actual real-world use, you can assume Class A AIS having about the same range as 25-watt FM Voice VHF Marine comms...
(figure 25 to 35 miles, for receiving Class A signals from large ships, line-of-sight comms...but if you have a rail-mounted AIS antenna, they're not likely to see your Class B AIS until closer...perhaps 15 - 20 miles...)

As for Class B AIS....since there are no real standards of antenna height / placement, cable loss, etc....Class B AIS "range" can be a bit of a crap-shoot....some systems work great, others don't!!
Sorry to be so "matter-of-fact", but the above is a fact!!

If you look at the above referenced thread on VHF and AIS radio range, you can easily see that even the low-powered (2-watt) Class B AIS transponder has plenty of transmitter power to attain the typically desired range....certainly limited by antenna height, just like Marine VHF-FM Voice!
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

If you want a general rule-of-thumb....figure Class B to Class B AIS range, for rail-mounted antennas (approx. 7.5' to 9' above the water), to be approx. 8 miles....some will be less, and some will be more....
But, figure about 8 miles...



5) Except for your AIS transponder's software possibly requiring a good SWR in order to continue to transmit, actually having an antenna "tuned to the AIS frequencies" is unnecessary....
But, it does make good engineering / good seamanship, to do so!

And, FYI....the more loss in your coaxial cable (whether because it is old, corroded connectors, smaller / higher-loss, etc.), the less important "tuning your antenna to AIS frequencies" is!!
As the added loss, actually improves the SWR seen at the transmitter end!

Understand that using a VHF antenna "tuned for the AIS frequencies" should not cause any issues on the primary VHF system....your VHF Voice comms will be fine...
Problem is, without the knowledge I posted in these other threads, and without actually testing these systems (both power and SWR, AND real-world use without any enhanced radiowave propagation conditions), many sailors will just assume that things are as they are supposed to be (even if they are not and their range is severely limited), and this is how you get such a wide range of opinions posted about "what works" and "what doesn't"....
But, if you read the facts, and do the tests, you will know what works and what doesn't....regardless what the internet myths say!



6) For those with "splitters", those considering a "splitter", and especially those who have had problems with them....
PLEASE read over this thread here!!

Vesper AIS SP-160 "relay/splitter" test results, lab/real world

Here you will learn more than you will ever need to know about use of a "splitter", allowing you to use your masthead VHF antenna for both your primary VHF radio and your AIS transponder....

And, if you had a splitter / masthead antenna system that worked worse than a rail-mounted antenna....you either had masthead antenna and/or coax issues, or had some enhanced radiowave propagation that influenced the results....
A VHF antenna at 65' height will have greater range than one at 10' height!!
Sorry again for being so blunt.....but, I've been doing this professional (and otherwise), for > 40 years....and these are the facts..


7) You cannot determine VHF and/or AIS range and performance based on what a "marina neighbor" observed....
a) A VHF vertical antenna has very deep nulls above and below, meaning it has VERY little signal straight down (and straight up), and as such someone that is very close, but with their antenna LOWER than yours, can be in one of these nulls, and someone one mile away would have a MUCH stronger signal...
(although, I doubt this is an issue for anyone)

b) Class B AIS sends out its dynamic data (position), when underway, at its quickest time...every 30 seconds.....but if standing still its much less often (I think it's every 3 minutes)...
BUT...
But, the static data (vessel name, callsigh, etc.) is only sent out every 6 minutes, no matter whether underway or at anchor...

So, it is VERY possible that a "marina neighbor" missed one (or two) static data transmissions (only transmitted every 6 minutes)....whether because his VHF and/or his AIS was in use during that time, or because your VHF was in use at that time, or because of him being in the deep null of your antenna....or more likely, a combination of all of these...

c) And, as I wrote above,
And, if you had a splitter / masthead antenna system that worked worse than a rail-mounted antenna....you either had masthead antenna and/or coax issues, or had some enhanced radiowave propagation that influenced the results....
A VHF antenna at 65' height will have greater range than one at 10' height!!
Sorry again for being so blunt.....but, I've been doing this professional (and otherwise), for > 40 years....and these are the facts..


Please have a look at this thread....
Vesper AIS SP-160 "relay/splitter" test results, lab/real world

Along with this one...
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

And, all of the myths / confusions will be understood!! (I promise!

Quote:
Originally Posted by trantor12020 View Post
Using regular vhf radio antenna on AIS Class B Transceiver, one could see AIS target as far away as 25nm or more. Would that target be able to see you?
Any idea any one?
Quote:
Originally Posted by Paul Elliott View Post
The answer is "Possibly, but probably not."

Were you receiving a Class-A signal? Those transmit at 25 watts, while a Class-B puts out 3 watts. Class-A generally transmits farther than Class-B. Both generally get line-of-sight performance, but Class-A usually does better.

The typical range for a Class-B signal is six miles minimum, perhaps twice that fairly often. However, under some radio propagation conditions both A and B signals can go hundreds or thousands of miles. While this is interesting when it happens, you can hardly depend on this type of range.
Quote:
Originally Posted by Weyalan View Post
We use our rather ordinary VHF aerial, mast mounted, with a splitter at the instrument panel. We pick up AIS from commercial shipping typically about 45 miles out, and often to 60-70 miles (if it is a big cargo vessel), so I don't think you need anything fancy
Quote:
Originally Posted by dannyjoh View Post
I initially set up a vesper marine transceiver and an antenna splitter for the mast-top VHF antenna. After hearing my marina neighbor saying that I occasionally disappear from his AIS screen 10-15 minutes at a time and suggesting a dedicated antenna, I installed a $50 AIS antenna on the stern rail. The difference was rather dramatic. Now I am seen more consistently, and I see way more boats on my AIS screen than I used to. So it does appear that a dedicated AIS antenna gives more efficient in transmitting/receiving AIS signal.

Quote:
Originally Posted by Sailorman Ed View Post
I had to buy a new antenna since I did not want to use a splitter and got a GAM antenna. It came with a nice little chart showing antenna length compared with frequency.
With these steps:
1) Remove the radiating element with a hex key.
2) Cut 3/4 of an inch off the bottom.
3) Reinsert the radiating element.
4) Tighten the hex screw.

Different but related question. When comparing distance with height of antenna, is it the bottom, center or top of the antenna for the height? I mounted mine to the dingy davits, base 7 feet off the water, top is 10 ft off.
I am not too worried about targets 100 miles away, just the ones between me and the horizon.
Quote:
Originally Posted by J Clark H356 View Post
Just have a receive unit - Digital Yacht AIS with wifi- and regularly get 30 to 40 NM with my masthead VHF antenna. When I was in Indian Key in southwest Florida, I was receiving ships near the Dry Tortugas showing ranges around 155 NM.

When my mast was down coming and going on the Tenn-Tom and Tennessee River to Mobile from Kentucky Lake, I mounted a Shakespeare AIS antenna on a 10 foot piece of PVC pipe u bolted to my stanchion. Both antenna setups go through a splitter. That put the antenna base 14 to 15 feet above the water. I typically picked up targets at around 8 miles when unobstructed by trees or terrain. I could not get good transmitting or receive on my regular VHF using this setup. I had to use my handheld into locks, communicating with tows and going into Marinas. My alternate antenna wire went into the same mast lead that my masthead antenna uses, so my assumption was that the AIS antenna wasn't good for normal VHF. I used a crimp connector, not soldered, so I suspect my installation was poor. Next time, I'll set it up in advance with an electronics installerI wonder if the AIS antenna wasn"t part of the problem. Does anybody use an AIS antenna tons it on VHF too?

I don't have an AIS transmitter, so have no experience with that side of AIS.
There is a LOT more to all of this...but, I think I hit the bullet-points!!!
(if I confused things, please let me know....and I will try to clarify!)

Fair winds...

John



P.S. Trantor, you should never need to do this....
Nothing wrong at all with a dedicated AIS antenna....they work, and work well for most...
But, you should never need to do what you did, in order to get the system to work...
Quote:
Originally Posted by trantor12020 View Post
YES! That's exactly what happened to my set.
Now I bypass the active Splitter just like you did.
So, I suspect that you do / did have another issue....especially troubling are intermittent coaxial cable connections, and especially those that show up only if the cable is positioned / curved in one specific angle, etc...

Just wish I knew more about your problem, and I would have helped!!
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Old 26-08-2016, 05:56   #32
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Re: AIS Antenna

Very helpful thread. Thanks.
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Old 26-08-2016, 06:25   #33
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Re: AIS Antenna

Quote:
Originally Posted by trantor12020 View Post
YES! That's exactly what happened to my set.
Now I bypass the active Splitter just like you did.
Coax length and quality really matters when it comes to receiving AIS signals. If you use RG58 coax and a masthead mounted antenna, the cable loss will be so high that the AIS will be close to useless. For any coax lengths longer than 2,m meter it is highly recommended to use RG214 coax or better.
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